The purpose of the proposed research is to investigate the development of spatial understanding: how children encode the locations of objects in space, in the period between 18 months and 6 years, and the uses they can rake of these representations in anticipating the outcomes of spatial transformations. The experiments are designed to address four questions: (1) whether early spatial coding includes metric information, and, if so, how metric accuracy changes or does not change with age; (2) whether early spatial coding uses single landmarks or a framework of landmarks, and whether the self is used as a landmark and, if so, under what conditions; (3) whether individual objects are internally coded in relation to each other; (4) under what conditions young children can compute the outcome of spatial transformations. The first two questions will be addressed using two methods: a sandbox hiding task suitable for examining coding of the location of a single object, and a free placement task with which the coding of arrays of objects can be examined. The question of whether these two tasks differ in representational demands will also be examined. The third question will be addressed using the free placement methodology alone. The fourth question will be addressed by comparing young children's ability to answer item questions about the outcome of viewer movement on the visual appearance of an array with their ability to answer appearance questions. The latter are the traditional Piagetian method for looking at perspective taking, while the former were shown by Huttenlocher and Presson (1979) to be much easier, at least for 9-year-old children. The proposed experiments should elucidate our understanding of spatial coding and transformation in very young children. The results would have implications for children with a variety of spatial handicaps, including blind children and children with learning disabilities. The results would also have implications for applied questions in the development of normal children, including the development of educational materials and environmental design.